1. Field of the Invention
The present invention relates to a diagnostic radiography system of the type having an X-ray generating unit for emitting an X-ray beam, a flat X-ray image converter that has a sensor unit with photosensitive pixel elements that are arranged in a matrix and an array which is arranged behind the sensor unit in the direction of radiation and LEDs that are arranged in a matrix as back-illumination, at least part of which capture the X-ray dose as a detector.
2. Description of the Prior Art
U.S. Pat. No. 5,757,884 describes this type of diagnostic radiography system with a flat X-ray converter having a scintillator layer and a semiconductor layer with photosensitive pixel elements that are arranged in a matrix as a sensor unit, and with an array, which is arranged behind this sensor unit in the direction of radiation, and LEDs that are arranged in a matrix as back-illumination, at least a part of which capture the X-ray dose as a detector.
In such solid state X-ray detectors which utilize amorphous semiconductors such as a-Si or a-Se, the back-illumination of the semiconductor structure is provided in order to improve the time behavior of the signal. The back-illumination typically consists of several sub-surfaces or modules which combine to form a compute surface large enough to illuminate all semiconductor pixel elements, for instance photodiodes. This illumination must be very homogeneous, since a non-homogeneous, chronologically unstable back-illumination leads to disturbing image artefacts.
An object of the present invention is to provide a back-illumination for a sensor unit of the type described above so as to produce a homogenous illumination which is chronologically stable.
The object is inventively achieved in an image converter wherein the elements of the back-illumination are connected to a control device, and when the back-illumination is activated the output signal of the sensor unit is measured, and the elements of the back-illumination are individually controlled by the control device for homogenization in the sense of a uniform output signal. With such measuring of the homogeneity of the back-illumination which is conducted frequently in the background, and with corresponding adjusting of the individual trigger powers of the elements, the illumination homogeneity can be improved. Here, the capability of the sensor unit to be sensitive not only to X-radiation from the front but also to visible light from the back is exploited.
It has proven advantageous for the elements of the back-illumination to be formed as modules with LEDs that are arranged in the form of a matrix.
The computing power in an image system that is already present in the system can be used when it is connected to the sensor unit for measuring its output signal.
A simple control is obtained when in an embodiment wherein a central unit detects when the X-ray generator is not active and emits a start signal to the image system and/or the control device in order to initiate and control the homogenization.
The image converter can advantageously employ and aSi:H detector.